This invention relates to single pass dehydrators in general and particularly concerns an improved dryer drum for such dehydrators which drum has central flighting structure supported in offset, thermally isolated relation to the drum sidewall for increasing the efficiency of the dehydrator.
In the past, it was conventional practice to force-dry alfalfa and similar crops immediately after cutting in order to eliminate potential damage to crops left in the field for sun curing. Moisture content of crops processed in this manner typically ranges as much as 78 percent such that the crops are able to withstand relatively high dryer inlet temperatures. Initially, dehydrators for drying these crops comprised a single pass system, the crop moving rectilinearly from a high temperature burner adjacent the inlet to the outlet of the dehydrator. Later, the more sophisticated three-pass system was developed wherein material is directed along a generally S-shape path of travel thereby providing desired long residence time in the dryer while at the same time requiring significantly less space than conventional single pass dryers.
As fuel costs have increased, it has become common practice to rely more heavily on the benefits of sun curing notwithstanding the risk of damage or loss resulting from leaving the cut crop in the field during the curing process. This technique appreciably reduces the moisture content of the crop presented to the dehydrator, and consequently correspondingly lowers the amount of fuel required to complete the drying process.
A problem with force-drying crop material which has been partially cured in the field is the fact that such material cannot withstand the high burner temperatures associated with the three-pass dehydration system used to process fresh cut crop. In this connection, the moisture content of field cured crops may be in the order of 13-15 percent which is simply not sufficient to protect the material against combustion under the influence of the high inlet temperatures in conventional three-pass systems. Moreover, merely reducing the inlet temperature of a three-pass dehydration system in order to process materials of lower moisture content would not be satisfactory inasmuch as the overall efficiency of the dehydration system would be unacceptably adversely affected.
Several attempts have been made to redesign the single pass dehydrator for efficient drying of low moisture content material, though none of these has been particularly successful. Typically the approach has been to increase the heat transfer surface and material residence time in the dryer by providing additional internal flighting structure. However, this approach has serious drawbacks from the stand-point of significantly increasing the initial cost of the dryer as well as presenting the problem with warpage of the flighting structure. In this latter regard, the flighting structure is typically supported on the dryer wall, the difference in thermal expansion between the flighting structure and the wall often resulting in unacceptable warpage of the structure. Of course, since the shape of the flighting structure is critical to the path of travel of the material through the dryer, deformation of the structure may significantly reduce the efficiency of the dehydration system.
One approach to the problem alluded to above has been the provision of central radiating flighting structure supported upon an elongate shaft mounted coaxially of the dryer drum. However, due to thermal stresses in the central flighting resulting from the manner in which it is supported upon the dryer drum, undesirable warpage and breakage has been experienced in the application of such dehydrators. Moreover, the long unsupported span presented by such structures requires the use of relatively expensive heavy-weight material to maintain the desired shape of the structure even under normal operating conditions.